The plug-type heating elements of the prior art consisted of a series of metal rectangular sections which have an insulator on each side of each section at a plurality of spaced locations along the length of the section. The electrical heating wire was wound around each of the spacers down one side of the frame and back up the opposite side of the frame.
One of the problems associated with the prior art heating element construction involves the space taken up by each frame member which space interferes with the flow of air through the plug unit. In other words, the heating elements when electrically connected provide a source of heat with the flow of air passing through and around the heat elements determining the quantity of heat emitted from the plug unit. If air flow is restricted by metal frame sections, the kilowatt output per unit area will be decreased causing loss of efficiency. Likewise, the use of multiple frame sections for a plug unit occupies a large percentage of the physical space which could be used for heating elements.
An additional drawback of the prior art is that the insulators which are attached to the frame sections are difficult to remove and each insulator must be removed from the frame section separately in order to change wire spacing or otherwise repair old plug units. Additionally, the cost of construction necessitated by the multiple frame sections and the attachment of each individual insulator to those frame sections grows increasingly with the cost of materials for the frames and insulators as well as the labor involved in assembling each plug unit.
Other examples of prior art use of electrical resistive elements do not solve the problems associated with plug type convection heating. For example, the patent to Eickemeyer, Jr. (U.S. Pat. No. 454,207) discloses an electrical resistor box which uses a frame structure A with a pair of rods M mounted thereon. Additionally, a plurality of spacers D are positioned along the rods with coil resistance elements E wound around the spacers between the rods M. However, Eickemeyer does not address itself to plug type convection heaters and its design is merely to provide a fire safe means for operating an electric elevator.
The patent of Jenkins (U.S. Pat. No. 716,048) discloses an electrical resistance heater comprising a frame-like casing H and including a pair of rods I, I' mounted therein. A plurality of grooved insulators G are mounted on the rods to support a coil resistance element A therebetween. The Jenkins patent is used as an electrical heating element which is not related to plug type convection heaters and employs a core inside of the coil wound wire thereby cutting down on the air flow through the entire structure making the Jenkins heater inappropriate for the plug type convection heaters.
The patent to Wilkinson et al (U.S. Pat. No. 1,602,804) which discloses an electrical resistance heater comprising a pair of rods 12 and 14 which are supported by walls 2 has each rod supporting a plurality of grooved ceramic spacers 18 mounted thereon. A coil resistance element 22 is wound around the spacer 18 between the rods 12 and 14. The Wilkinson patent is not related to air flow convection type heaters and is primarily used as discussed in the patent in the form of electric toasters or other such appliances whose main object is radiated heat in a confined space.
Atkinson et al (U.S. Pat. No. 2,155,289) discloses an electrical resistance heating comprising a frame 1 and 2 which supports a pair of rods 3. The rods, in turn, support a plurality of grooved ceramic spools 7. A coil resistance element 8 is wound around the ceramic spools between the rods 3. The heater of Atkinson is not set up in a manner so as to provide for a plug type convection heating system and additionally the wire is continuously wound in a coil-shaped manner throughout the entire structure. The main purpose of the heater of Atkinson is to provide a radiating type of electrical space heater as shown in FIG. 7.
The patent to Weyenberg (U.S. Pat. No. 3,212,045) discloses a grid resistor comprising a pair of frame members 1 separated by a pair of threaded rods 2. Metallic resistance elements 5 are spaced apart by a plurality of insulated spacers 9 and sleeves 13 mounted around the rods 2. The Weyenberg heater uses a metallic resistance element 5 which is unrelated to coiled resistance wires and furthermore the design features of the grid resistance show that it is not meant to be used as a convection type of plug heating unit because of the shape of the resistive metallic elements.
The patents to Best (U.S. Pat. No. 4,016,403), Beck (U.S. Pat. No. 4,011,395), Paulson et al (U.S. Pat. No. 3,883,721), Dunlop (U.S. Pat. No. 1,726,476), Simmons (U.S. Pat. No. 1,698,282) and Colby (U.S. Pat. No. 1,335,483) generally show various types of open framework electrical resisting heating units which provide for adequate space heating but are not designed to alleviate the problems concerned with minimizing space to permit greater air flow thereby increasing the BTU output of plug type heater units.